An ignition coil for an internal combustion engine that is installed directly on an engine and that is directly coupled with spark plugs is well known. Various configurations of such ignition coils for internal combustion engines have been proposed to achieve compactness and reduced weight. The ignition coil of the prior art is filled up around a coil portion, as fitted in a housing, with a thermoset resin (molding resin) such as an epoxy resin to prevent the high voltage generated by the coil portion from leaking out of the housing and causing a dielectric breakdown in the coil by the high voltage. Considering the adhesion between the inner wall of the housing and the molding resin, therefore, it is known to make the housing of a thermoplastic polyester resin such as polybutylene terephthalate (PBT) or polyethylene terephthalate (PET).
A device employing insulation material of insulating oil, epoxy resin, or the like to ensure insulation against high voltage is known, and the use of silicone rubber to encapsulate the coil is known.
However, demand for high output and high efficiency is increasing because the cylinder-head portions of engines are increasing in complexity because of adoption of more valves and improvements in combustion-chamber configuration. Space constraints for installation of the ignition coil is an increasing problem and concern. In the case of a DOHC engine in particular, increasingly narrower valve parting angles are being attempted, and the state is such that installation of a thick ignition coil is extremely difficult, and suitable dimensions which are housable within for example a plug hole are necessary.
Moreover, because the ignition coil is placed in the high-temperature environment immediately proximately to the engine body according to the foregoing prior art, there is a problem in which the insulating material is susceptible to temperature degradation. In particular, when the interior of the case is filled completely with insulating resin or oil, there is a problem whereby high-voltage durability declines due to oxidation and degradation of these materials.